This invention relates to a method for measuring the physical-chemical kinetics of resin systems undergoing polymerization. Specifically, it relates to a method for monitoring the extent and determining the rate of cure of resin systems undergoing polymerization by incorporating into the resin system a fluorescent dye which reacts with the resin and changes the fluorescence in proportion to the extent of cure of the resin.
Polymerized resin systems are widely used in this and foreign countries for a variety of purposes. In general, any plastic structure, whether thermoplastic or thermosetting, is derived from a polymerized resin system.
Epoxy resins, for example, are commercially used in coating and structural applications. They are frequently used as fiber-reinforcing materials in the production of laminates and composites. Printed wiring boards used in electronic devices, in particular, are made from epoxy resin-based laminates or composites. Epoxies are used because of their hardness, resistance to corrosive environments, and relatively low dielectric constants.
The quality and properties of composite parts made from epoxy resins depend on several factors, including the proper cure of the epoxy resin. Variations in time and temperature, and polymer composition all affect the proper curing. Typically, the composites are cured for excessive lengths of time to insure the complete cure of the epoxy resin. The ability to monitor the epoxy curing process on line for the fabrication of composite parts would provide several advantages over existing methods. Correct cure cycles could be identified to prevent incomplete cure of the epoxy resin. Incomplete cure results in a loss of strength of the composite, a lack of uniformity in the physical characteristics, and undesirable variations in the dielectric constant. It also affects the thermal stability of the end product. The ability to identify correct cure cycles is especially important in the production of high-cost parts, where increasing the production rate while maintaining quality would be valuable In addition, part quality could be inspected by randomly sampling the extent of epoxy cure. Finally, the cure could be monitored in different regions of large or complex parts to insure homogeneous properties in the final product.
Epoxy resins are characterized by a 3-membered ring known as the epoxy, epoxide, oxirane, or ethoxylene group. These resins are prepared by the reaction of compounds containing an active hydrogen group with epichlorohydrin, followed by dehydrohalogenation. Optimum performance properties are obtained by cross-linking the epoxy resins into a three-dimensional insoluble and infusible network To accomplish this, the resin is treated with a curing agent or hardener The specific choice of curing agent or hardener depends on processing methods, curing conditions, and the specific physical and chemical properties desired. Primary and secondary amines are the most widely used curing agents for epoxy resins.
The structure and properties of epoxies are known to strongly depend on the extent of cure and physical aging which has taken place after the cure cycle is completed. The curing of a thermoset epoxy resin can be expressed in terms of a time-temperature-transformation relationship. There are normally considered to be four distinct states of the thermosetting-curing process: liquid, gelled rubber, ungelled glass, and gelled glass. The time-temperature-transformation diagrams can be used to establish chemical structure-physical property relationships of fully cured systems, since each system is unique in the kinetic before final cure and the physical properties it imparts. The extent of cross-linking is a measure of the degree of cure. The most favorable properties, i.e., high strength, thermal stability, chemical resistance, etc., are obtained by maximum cross-linking.